The present invention relates to both a method and a control unit for setting a turbine flow cross-section of the turbocharger in a motor vehicle in the event of a change of the load of the gasoline engine from a smaller load value to a larger load value. Because of the time derivative of the load, which is then positive, such a load change may also be referred to as a positive load change.
Such a method is known from the series “Die Bibliothek der Technik, Band 103, Abgasturbolader [The Library of Technology, Volume 103, Exhaust Gas Turbochargers]”, Verlag Moderne Industrie, D-86895 Landsberg/Lech, ISBN 3-478-93263-7, page 40. This literature citation relates to a turbocharger having variable turbine geometry (VTG), in which the turbine flow cross-section is reduced by closing blades to generate a higher pressure gradient between the turbine intake and the turbine outlet. To accelerate out of low speeds, the blades are to be closed in order to obtain the maximum energy from the exhaust gas. Furthermore, the blades open with increasing speed and adapt themselves to the particular operating point.
According to this publication, increasing the torque at low speeds under stationary conditions and tangible improvement of the dynamic driving behavior in passenger automobile diesel engines is particularly to be achieved. The variable turbine geometry particularly contributes to reducing the fundamental delay between an increased torque demand and the provision of the increased torque by the internal combustion engine, known as turbo lag. A use of turbocharger having variable turbine geometry in gasoline engines is also already mentioned in the cited publication.
This turbo lag also represents a challenge for the developers in gasoline engines. Although gasoline engines have been operated up to this point using turbochargers which have fixed turbine geometry because of higher exhaust gas temperatures, one also expects positive effects in principle on the undesired turbo lag effect if turbochargers having variable turbine geometry are used in gasoline engines. The positive effects observed in tests remain, however, behind the expectations.
On this basis, an object of the present invention is to provide a method and a control unit in which an accelerated torque build-up upon a positive load change may be achieved.
This object has been achieved in a method and a control unit according to the present invention by using a control unit and method in which an actuator for setting the turbine flow cross-section to be set for the larger load value is activated with a delay in relation to the change of the load.
It is surprising at first glance that a delayed actuator activation actually accelerates the torque build-up during a positive load change. However, the inventors have recognized that the pressure build-up in the exhaust gas before the turbine, which is desirable per se, may have a counterproductive effect in the event of an undelayed activation of the actuator in a gasoline engine. The reason is that, in contrast to the diesel engine which is typically operated unthrottled, gasoline engines are operated throttled in the part-load range.
The diesel engine operates in part load using excess air. The desired torque is set via the metered fuel quantity. Therefore, it is set via the quality of the combustion chamber charge in the event of essentially identical combustion chamber charge. As a result, the diesel engine delivers a comparatively large exhaust gas mass flow, which keeps the turbine at speed, even in the part-load range, in which it only generates little torque. A closing activation of the blade adjustment then results practically without delay in an increased turbine output, an increased boost pressure, and thus a rapid increase of the combustion chamber charges and the torque of the diesel engine.
In contrast, the power of the gasoline engine is set via the quantity of the mixture combusted in the combustion chamber. At low power, i.e., at low torques and/or speeds, only a correspondingly smaller exhaust gas mass flow also results. In a specific internal combustion engine, the air mass intake per hour varies between 24 kg and 1400 kg depending on load, i.e., by almost two orders of magnitude. In operating states having low exhaust gas mass flows, the turbine speed thus drops comparatively strongly. A closing adjustment of the blades increases the relative proportion of the exhaust gas energy transmitted to the turbine. However, because of the exhaust gas mass flow, which is low in absolute value, a large increase of absolute turbine power is not yet connected thereto. As an undesired result, the boost pressure only rises slowly. In addition, the exhaust gas counterpressure, which arises due to the closing adjustment of the blades, may react on the combustion chambers and obstruct their charge with uncombusted air or uncombusted mixture.
In contrast, the exhaust gas mass flow may first be increased with open throttle valve by the delay of the actuator activation according to the present invention. When the closing adjustment occurs after passage of the short delay time, an increased exhaust gas mass flow is already available, so that the exhaust gas energy transmitted to the turbine is significantly greater than in the event of an undelayed adjustment. As a result, the torque provided by the gasoline engine rises significantly faster than with the undelayed activation. The same advantages result for corresponding embodiments of the control unit.